The present invention relates to a pneumatic tire. More particularly, it relates to a pneumatic tire having excellent conductivity that has a tread of silica formulation or the like, improves rolling resistance and wet performance of a tire and can discharge static electricity charged in vehicles through a road surface.
To improve rolling resistance of a pneumatic tire and running performance (wet performance) on wet pavement, a technique of compounding silica in place of the conventional carbon black as a reinforcing agent with a rubber composition of a tread. With this silica compounding technique, discharge phenomenon occurs by static electricity charged in vehicles when a tire passes on a manhole or the like, and this gives rise to the problems of radio noise, adverse influence to electronic circuit parts, generation of short-circuit and the like.
To overcome those problems, various techniques of ensuring conductivity of a tire by providing a conductive member having compounded therewith carbon black on a part of a tread structure have conventionally been proposed. For example, the technique disclosed in JP-A-2002-1834 (kokai) (the entire contents of this reference being incorporated herein by reference) is to form a conductive thin film containing carbon black on an outer surface of a tread and a side wall by applying a conductive liquid rubber paste composition to an area of from the vicinity of the part corresponding to the ground end of a tread of a green tire to the part corresponding to a buttress, vulcanizing and molding, thereby covering the entire groove surface of transverse grooves at a tire shoulder.
The technique described in EP 0 819 741 A1 (the entire contents of this reference being incorporated herein by reference) is to form a continuous coating film by applying a rubber cement obtained by dissolving and uniformly dispersing a rubber composition comprising 100 parts by weight of a diene rubber and from 40 to 100 parts by weight of carbon black having an N2SA of 130 m2/g or more and a DBP absorption of 110 ml/100 g or more in an organic solvent, to an outer surface of a tire tread cap rubber having an intrinsic resistance value of 108 Ω·cm or more and a part of at least one member adjacent to the outer surface.
The technique described in EP 0 705 722 A1 (the entire contents of this reference being incorporated herein by reference) is to have a carbon black-reinforced carcass and a rubber tread quantitatively reinforced with silica, the tread having thereon a protective thin coating film containing a given amount of an electrically conductive carbon black.
U.S. Pat. No. 6,140,407 (the entire contents of this reference being incorporated herein by reference) describes that an aqueous conductive coating containing a rubber component, carbon black having a nitrogen absorption method specific surface area (N2SA) of from 70 to 180 m2/g and a dibutyl phthalate (DBP) absorption of from 70 to 180 ml/100 g, and a surfactant is applied over the surface of a tread constituted of a rubber composition having high electric resistance and the surface of a side wall constituted of a rubber composition having low electric resistance adjacent to the tread.
JP-A-2005-2206 (kokai) (the entire contents of this reference being incorporated herein by reference) describes a rubber composition comprising a rubber component containing a diene rubber, having compounded therewith a carbon black component containing a highly conductive carbon black or acetylene black having a specified specific surface area, and other carbon black, the rubber composition after vulcanization having a loss tangent (tan δ) of from 0.03 to 0.07 at 60° C. and a volume resistivity of from 104 to 106 Ω·cm at 25° C., thereby providing a pneumatic tire having both low rolling resistance and high antistatic property.
However, the techniques described in the above references do not reach to overcome the improvement of low rolling resistance and wet performance by compounding a non-conductive filler such as silica and the problem of non-conductivity of a tire based on a non-conductive tread, in combination. In other words, even though conductivity was improved by providing a conductive thin film comprising a diene rubber containing carbon black having a large specific surface area on the surface of a tire, where carbon black having a large specific area is contained in an amount of 40 parts by weight or more, there were the problems such that the rubber composition becomes highly exothermic, and satisfactory low rolling resistance cannot be achieved; unvulcanization viscosity is increased due to the decrease of dispersibility of carbon black, and this adversely affects processability in production process; and rubber strength of a conductive thin film is decreased with tire running, and conductive performance cannot be sustained over a long period of time.